[GitHub] [arrow-rs] tustvold commented on a diff in pull request #2781: Add i256 (#2637)

[GitBox <gi...@apache.org>]

tustvold commented on code in PR #2781:
URL: https://github.com/apache/arrow-rs/pull/2781#discussion_r985522101


##########
arrow-buffer/src/bigint.rs:
##########
@@ -0,0 +1,326 @@
+// Licensed to the Apache Software Foundation (ASF) under one
+// or more contributor license agreements.  See the NOTICE file
+// distributed with this work for additional information
+// regarding copyright ownership.  The ASF licenses this file
+// to you under the Apache License, Version 2.0 (the
+// "License"); you may not use this file except in compliance
+// with the License.  You may obtain a copy of the License at
+//
+//   http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing,
+// software distributed under the License is distributed on an
+// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+// KIND, either express or implied.  See the License for the
+// specific language governing permissions and limitations
+// under the License.
+
+use num::BigInt;
+use std::cmp::Ordering;
+
+/// A signed 256-bit integer
+#[allow(non_camel_case_types)]
+#[derive(Copy, Clone, Default, Eq, PartialEq, Hash)]
+pub struct i256 {
+    low: u128,
+    high: i128,
+}
+
+impl std::fmt::Debug for i256 {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        write!(f, "{}", self)
+    }
+}
+
+impl std::fmt::Display for i256 {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        write!(f, "{}", BigInt::from_signed_bytes_le(&self.to_le_bytes()))
+    }
+}
+
+impl PartialOrd for i256 {
+    fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
+        Some(self.cmp(other))
+    }
+}
+
+impl Ord for i256 {
+    fn cmp(&self, other: &Self) -> Ordering {
+        // This is 25x faster than using a variable length encoding such
+        // as BigInt as it avoids allocation and branching
+        self.high.cmp(&other.high).then(self.low.cmp(&other.low))
+    }
+}
+
+impl i256 {
+    /// The additive identity for this integer type, i.e. `0`.
+    pub const ZERO: Self = i256 { low: 0, high: 0 };
+
+    /// The multiplicative identity for this integer type, i.e. `1`.
+    pub const ONE: Self = i256 { low: 1, high: 0 };
+
+    /// The multiplicative inverse for this integer type, i.e. `-1`.
+    pub const MINUS_ONE: Self = i256 {
+        low: u128::MAX,
+        high: -1,
+    };
+
+    /// Create an integer value from its representation as a byte array in little-endian.
+    #[inline]
+    pub fn from_le_bytes(b: [u8; 32]) -> Self {
+        Self {
+            high: i128::from_le_bytes(b[16..32].try_into().unwrap()),
+            low: u128::from_le_bytes(b[0..16].try_into().unwrap()),
+        }
+    }
+
+    /// Create an i256 from the provided low u128 and high i128
+    #[inline]
+    pub fn from_parts(low: u128, high: i128) -> Self {
+        Self { low, high }
+    }
+
+    /// Return the memory representation of this integer as a byte array in little-endian byte order.
+    #[inline]
+    pub fn to_le_bytes(self) -> [u8; 32] {
+        let mut t = [0; 32];
+        let t_low: &mut [u8; 16] = (&mut t[0..16]).try_into().unwrap();
+        *t_low = self.low.to_le_bytes();
+        let t_high: &mut [u8; 16] = (&mut t[16..32]).try_into().unwrap();
+        *t_high = self.high.to_le_bytes();
+        t
+    }
+
+    /// Create an i256 from the provided [`BigInt`] returning a bool indicating
+    /// if overflow occurred
+    fn from_bigint_with_overflow(v: BigInt) -> (Self, bool) {
+        let v_bytes = v.to_signed_bytes_le();
+        match v_bytes.len().cmp(&32) {
+            Ordering::Less => {
+                let mut bytes = if num::Signed::is_negative(&v) {
+                    [255_u8; 32]
+                } else {
+                    [0; 32]
+                };
+                bytes[0..v_bytes.len()].copy_from_slice(&v_bytes[..v_bytes.len()]);
+                (Self::from_le_bytes(bytes), false)
+            }
+            Ordering::Equal => (Self::from_le_bytes(v_bytes.try_into().unwrap()), false),
+            Ordering::Greater => {
+                (Self::from_le_bytes(v_bytes[..32].try_into().unwrap()), true)
+            }
+        }
+    }
+
+    /// Performs wrapping addition
+    #[inline]
+    pub fn wrapping_add(self, other: Self) -> Self {
+        let (low, carry) = self.low.overflowing_add(other.low);
+        let high = self.high.wrapping_add(other.high).wrapping_add(carry as _);
+        Self { low, high }
+    }
+
+    /// Performs checked addition
+    #[inline]
+    pub fn checked_add(self, other: Self) -> Option<Self> {
+        let (low, carry) = self.low.overflowing_add(other.low);
+        let high = self.high.checked_add(other.high)?.checked_add(carry as _)?;
+        Some(Self { low, high })
+    }
+
+    /// Performs wrapping subtraction
+    #[inline]
+    pub fn wrapping_sub(self, other: Self) -> Self {
+        let (low, carry) = self.low.overflowing_sub(other.low);
+        let high = self.high.wrapping_sub(other.high).wrapping_sub(carry as _);
+        Self { low, high }
+    }
+
+    /// Performs checked subtraction
+    #[inline]
+    pub fn checked_sub(self, other: Self) -> Option<Self> {
+        let (low, carry) = self.low.overflowing_sub(other.low);
+        let high = self.high.checked_sub(other.high)?.checked_sub(carry as _)?;
+        Some(Self { low, high })
+    }
+
+    /// Performs wrapping multiplication
+    #[inline]
+    pub fn wrapping_mul(self, other: Self) -> Self {
+        let (low, high) = mulx(self.low, other.low);
+
+        // Compute the high multiples, only impacting the high 128-bits
+        let hl = self.high.wrapping_mul(other.low as i128);
+        let lh = (self.low as i128).wrapping_mul(other.high);
+
+        Self {
+            low,
+            high: (high as i128).wrapping_add(hl).wrapping_add(lh),
+        }
+    }
+
+    /// Performs checked multiplication
+    #[inline]
+    pub fn checked_mul(self, other: Self) -> Option<Self> {
+        let (low, high) = mulx(self.low, other.low);
+
+        // Compute the high multiples, only impacting the high 128-bits
+        let hl = self.high.checked_mul(other.low as i128)?;
+        let lh = (self.low as i128).checked_mul(other.high)?;
+
+        Some(Self {
+            low,
+            high: (high as i128).checked_add(hl)?.checked_add(lh)?,
+        })
+    }
+
+    /// Performs wrapping division
+    #[inline]
+    pub fn wrapping_div(self, other: Self) -> Self {
+        let l = BigInt::from_signed_bytes_le(&self.to_le_bytes());
+        let r = BigInt::from_signed_bytes_le(&other.to_le_bytes());
+        Self::from_bigint_with_overflow(l / r).0
+    }
+
+    /// Performs checked division
+    #[inline]
+    pub fn checked_div(self, other: Self) -> Option<Self> {
+        let l = BigInt::from_signed_bytes_le(&self.to_le_bytes());
+        let r = BigInt::from_signed_bytes_le(&other.to_le_bytes());
+        let (val, overflow) = Self::from_bigint_with_overflow(l / r);
+        (!overflow).then(|| val)
+    }
+
+    /// Performs wrapping remainder
+    #[inline]
+    pub fn wrapping_rem(self, other: Self) -> Self {
+        let l = BigInt::from_signed_bytes_le(&self.to_le_bytes());
+        let r = BigInt::from_signed_bytes_le(&other.to_le_bytes());
+        Self::from_bigint_with_overflow(l % r).0
+    }
+
+    /// Performs checked remainder
+    #[inline]
+    pub fn checked_rem(self, other: Self) -> Option<Self> {
+        if other == Self::ZERO {
+            return None;
+        }
+
+        let l = BigInt::from_signed_bytes_le(&self.to_le_bytes());
+        let r = BigInt::from_signed_bytes_le(&other.to_le_bytes());
+        let (val, overflow) = Self::from_bigint_with_overflow(l % r);
+        (!overflow).then(|| val)
+    }
+}
+
+#[inline]
+fn mulx(a: u128, b: u128) -> (u128, u128) {
+    let split = |a: u128| (a & (u64::MAX as u128), a >> 64);
+
+    const MASK: u128 = u64::MAX as _;
+
+    let (a_low, a_high) = split(a);
+    let (b_low, b_high) = split(b);
+
+    // Carry stores the upper 64-bits of low and lower 64-bits of high
+    let (mut low, mut carry) = split(a_low * b_low);
+    carry += a_high * b_low;
+
+    // Update low and high with corresponding parts of carry
+    low += carry << 64;
+    let mut high = carry >> 64;
+
+    // Update carry with overflow from low
+    carry = low >> 64;
+    low &= MASK;
+
+    // Perform multiply including overflow from low
+    carry += b_high * a_low;
+
+    // Update low and high with values from carry
+    low += carry << 64;
+    high += carry >> 64;
+
+    // Perform 4th multiplication
+    high += a_high * b_high;
+
+    (low, high)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use num::BigInt;
+    use rand::{thread_rng, Rng};
+
+    #[test]
+    fn test_signed_cmp() {
+        let a = i256::from_parts(i128::MAX as u128, 12);
+        let b = i256::from_parts(i128::MIN as u128, 12);
+        assert!(a < b);
+
+        let a = i256::from_parts(i128::MAX as u128, 12);
+        let b = i256::from_parts(i128::MIN as u128, -12);
+        assert!(a > b);
+    }
+
+    #[test]
+    fn test_i256() {
+        let mut rng = thread_rng();
+
+        for _ in 0..1000 {
+            let mut l = [0_u8; 32];
+            let len = rng.gen_range(0..32);
+            l.iter_mut().take(len).for_each(|x| *x = rng.gen());
+
+            let mut r = [0_u8; 32];
+            let len = rng.gen_range(0..32);
+            r.iter_mut().take(len).for_each(|x| *x = rng.gen());
+
+            let il = i256::from_le_bytes(l);
+            let ir = i256::from_le_bytes(r);
+
+            let bl = BigInt::from_signed_bytes_le(&l);
+            let br = BigInt::from_signed_bytes_le(&r);
+
+            // Comparison
+            assert_eq!(il.cmp(&ir), bl.cmp(&br), "{} cmp {}", bl, br);
+
+            // Addition
+            let actual = il.wrapping_add(ir);
+            let (expected, overflow) =
+                i256::from_bigint_with_overflow(bl.clone() + br.clone());
+            assert_eq!(actual, expected);
+
+            let checked = il.checked_add(ir);
+            match overflow {
+                true => assert!(checked.is_none()),
+                false => assert_eq!(checked.unwrap(), actual),
+            }
+
+            // Subtraction
+            let actual = il.wrapping_sub(ir);
+            let (expected, overflow) =
+                i256::from_bigint_with_overflow(bl.clone() - br.clone());
+            assert_eq!(actual.to_string(), expected.to_string());
+
+            let checked = il.checked_sub(ir);
+            match overflow {
+                true => assert!(checked.is_none()),
+                false => assert_eq!(checked.unwrap(), actual),
+            }
+
+            // Multiplication
+            let actual = il.wrapping_mul(ir);
+            let (expected, overflow) =
+                i256::from_bigint_with_overflow(bl.clone() * br.clone());
+            assert_eq!(actual.to_string(), expected.to_string());
+
+            let checked = il.checked_mul(ir);
+            match overflow {
+                true => assert!(checked.is_none()),
+                false => assert_eq!(checked.unwrap(), actual),
+            }

Review Comment:
   Yeah I figured that would be a bit circular



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